ORCID Profile
0000-0003-3095-9856
Current Organisations
Universitat de Barcelona
,
University of Reading
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Publisher: Wiley
Date: 16-11-2022
Publisher: Copernicus GmbH
Date: 28-11-2022
DOI: 10.5194/WCD-2022-59
Abstract: Abstract. Atmospheric blocking is a circulation pattern that describes the presence of large-scale, persistent anticyclones, which have the potential to bring severe impacts at the surface. However, the dynamical behaviour of blocks is still not fully understood. For ex le, the factors that determine the persistence of blocking events are not clear. In this study, the relationship between blocks and smaller-scale transient anticyclonic eddies is examined, with a particular focus on the impact of transients on the persistence of a block. Analysis is performed in two areas: the Euro-Atlantic and North Pacific, which are locations with both high blocking frequency and potential for severe impacts. Geopotential height anomalies at 500 hPa are used to identify blocking events and the anticyclonic transient eddies. This allows for a Eulerian definition of blocking, as well as a Lagrangian perspective on the eddies. It is found that anticyclonic eddies experience a northward acceleration prior to entering a block, which is indicative of ridge-building ahead of a block, but could also potentially provide evidence for the previously-proposed Selective Absorption Mechanism for block maintenance. A general pattern is found whereby longer blocks interact with more anticyclonic transients than less persistent blocks at all times of year. This effect is strongest in winter and weakest in summer, which agrees with the fact that blocks are most persistent in winter and least persistent in summer. However, the strength of the anticyclonic eddy, measured by its maximum 500 hPa geopotential height anomaly, that interacts with a block generally has very little bearing on the persistence of a block, aside from a few cases.
Publisher: Copernicus GmbH
Date: 28-06-2022
DOI: 10.5194/EMS2022-272
Abstract: & & Atmospheric blocking is often responsible for high-impact surface weather conditions such as heatwaves, cold spells, and droughts. Very long blocking events are particularly problematic due to the persistence of hazardous surface conditions. Despite their many impacts, understanding the dynamical behaviour of blocking events is still incomplete despite recent progress. Here, the relationship between block persistence and anticyclonic eddies contributing towards these blocks is investigated. Anticyclonic eddies that contribute to blocking are defined as large positive 500 hPa geopotential height (Z500) anomalies, obtained by filtering in both space and time, that pass through a blocked region. These features are then followed using an objective feature tracking algorithm. It emerges that stationary blocking conditions are often the result of more than one transient anticyclonic eddy entering the block itself, which is a reflection of the selective absorption mechanism (SAM) for block maintenance, proposed by Yamazaki and Itoh (2013). A relationship is found between the number of anticyclonic eddies contributing to a block and the persistence of the block itself, with longer-lasting blocks typically absorbing more eddies than less persistent events this behaviour is particularly noticeable in winter. The contribution of the smaller eddies to the blocks via the SAM is also observed as the anomalies speed up slightly just before entering the blocking region, before intensifying, becoming slow-moving, and sometimes reversing in direction inside the block itself. In addition to this climatological viewpoint, case studies have also been analysed to obtain a more detailed view of the process. From these, it is observed that some of the Z500 anomalies that contribute to blocking events originate from a long way upstream and travel along the wave guide until they are absorbed into a block, and this is again most evident in winter. The results from this work suggest there is an inherent link between repeated block maintenance and the persistence of block events, and also provides evidence that block maintenance processes may differ according to time of year.& &
Publisher: Wiley
Date: 10-2022
DOI: 10.1002/QJ.4367
Abstract: Variations in the character of monsoonal rainfall over the Western Ghats region on the west coast of India are studied using radiosondes, satellite observations, and reanalysis products. Summer monsoon rainfall over this region occurs in alternate offshore and onshore phases. It is shown that these phases are controlled primarily by the strength of the low‐level westerly jet. Thus, a classification based on the Froude number, , of the onshore flow is proposed, where, is the mountain height, is the mean wind speed, and is the mean Brunt–Väisäla frequency over depth . At low ( 0.5), onshore winds are weak and the diurnal thermal fluctuation over the orography is strong the land–sea and mountain–valley circulations are enhanced, leading to a stronger diurnal control over the rainfall. A nocturnal offshore propagation of rainfall from the west coast is seen during this phase. Rainfall over the rainshadow region to the east of the Western Ghats also increases, due to a weaker lee effect, while it decreases over the Western Ghats, due to a greater blocking effect. At high ( 1), orographic blocking of the low‐level winds is weak. Thus, rainfall is enhanced over the Western Ghats and reduced over the rainshadow region due to a stronger lee effect. In this phase, the diurnal thermal fluctuation over the orography is weak. The bulk Richardson number is less than 1, suggesting a dominance of vertical wind shear over the buoyancy forces. The level of free convection and convective inhibition over the west coast are also very low. Hence, at high , rainfall over the west coast results mainly from mechanical uplifting of the westerly winds by the Western Ghats, with no preference for a particular time of day. These findings will help in improving the representation of orographic effects and the diurnal cycle of rainfall in numerical models.
Publisher: Copernicus GmbH
Date: 08-08-2023
Abstract: Abstract. Atmospheric blocking is a circulation pattern that describes the presence of large-scale, persistent anticyclones, which have the potential to bring severe impacts at the surface. However, the dynamical behaviour of blocks is still not fully understood. For ex le, the factors that determine the persistence of blocking events are not clear. In this study, the relationship between blocks and smaller-scale transient anticyclonic eddies is examined, with a particular focus on the impact of transients on the persistence of a block. Analysis is performed in two areas: the Euro-Atlantic and North Pacific, which are locations with both high blocking frequency and potential for severe impacts. Geopotential height anomalies at 500 hPa are used to identify blocking events and the anticyclonic transient eddies. This allows for a Eulerian definition of blocking, as well as a Lagrangian perspective on the eddies. It is found that anticyclonic eddies experience a northward acceleration prior to entering a block, which is indicative of ridge building ahead of the block but could also potentially provide evidence for the previously proposed selective absorption mechanism for block maintenance. A general pattern is found whereby longer blocks interact with more anticyclonic transients than less persistent blocks at all times of the year. This effect is strongest in winter and weakest in summer, which agrees with the fact that blocks are most persistent in winter and least persistent in summer. However, the strength of the anticyclonic eddy that interacts with a block, measured by its maximum 500 hPa geopotential height anomaly, has a more complicated relationship with block persistence. The strength of anticyclonic transient eddies is a more determining factor of block persistence in the North Pacific than in the Euro-Atlantic region. In the North Pacific the longest blocks interact with stronger eddies than the shortest blocks in all seasons except summer, when the reverse is true. By contrast, longer Euro-Atlantic blocks only result from stronger anticyclonic eddies in autumn and winter. We therefore conclude that the number of anticyclonic eddies that interact with a block is most important in determining its persistence, with the strength of the eddies having a more variable effect.
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15444
Abstract: & & Precipitation distribution around an orographic barrier is controlled by the Froude Number (Fr) of the impinging flow. Fr is essentially a ratio of kinetic energy and stratification of winds around the orography. For Fr & 1 (Fr & ), the flow is unblocked (blocked) and precipitation occurs over the mountain peaks and the lee region (upwind region). While idealized modelling studies have robustly established this relationship, its widespread real-world application is h ered by the dearth of relevant observations. Nevertheless, the data collected in the field c aigns give us an opportunity to explore this relationship and provide a testbed for numerical models. A realistic distribution of precipitation over a mountainous region in these models is necessary for flash-flood and landslide forecasting. The Western Ghats region is a classic ex le where the orographically induced precipitation leads to floods and landslides during the summer monsoon season. In the recent INCOMPASS field c aign, it was shown that the precipitation over the west coast of India occurred in alternate offshore and onshore phases. The Western Ghats received precipitation predominantly during the onshore phase which was characterized by a stronger westerly flow. Here, using the radiosonde data from a station over the Indian west coast and IMERG precipitation product, we show that climatologically, these phases can be mapped over an Fr-based classification of the monsoonal westerly flow. Classifying the flow as 'High Fr' (Fr & ), 'Moderate Fr' ( 0.5 & Fr & #8804 1) and 'Low Fr' ( Fr & #8804 0.5 ) gives three topographical modes of precipitation -- 'Orographic', 'Coastal' and 'Offshore', respectively. & Moreover, these modes are not sensitive to the choice of radiosonde station over the west coast.& &
Publisher: American Geophysical Union (AGU)
Date: 07-07-2020
DOI: 10.1029/2019JD032184
Publisher: American Geophysical Union (AGU)
Date: 02-2018
DOI: 10.1002/2017MS001115
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-12203
Abstract: & & A full understanding of the dynamical behaviour of atmospheric blocking is still lacking, despite the influence of blocking towards hazardous mid-latitude weather extremes. Using geopotential height anomalies, relative to the zonal mean flow and persistent longitude-dependent eddies, and an objective feature tracking algorithm, the climatologies and lifecycles of anticyclones that contribute to blocking events are explored. Case studies and a climatology for blocking are presented using this process, and results show that this method performs favourably in relation to existing block detection methods since most blocking configurations are successfully detected. Then, blocking events are classified according to location of occurrence and persistence, and characteristics including intensity and areal extent are examined. The anticyclonic features contributing to blocks are also studied in terms of their genesis and lysis regions, along with anomaly strength and speed. It is found that many of the anticyclonic features that enter a block form a long way upstream, before travelling along the Rossby wave guide and intensify in the block. Furthermore, anticyclonic features that leave a block can then proceed to re-intensify further downstream and be part of a further blocking episode in a new location. This shows that there is an inherent interaction between transient waves and stationary blocks, and these results provide evidence for the previously-proposed selective absorption mechanism (SAM) for block maintenance.& &
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-15748
Abstract: & & IMPROVE is motivated by the effects of orography on Indian precipitation as part of the diurnal cycle of convection, contributing to water supply, as well as its role in extreme events.& IMPROVE considers two focal regions.& The Western Ghats, which intercept the monsoon flow across the Arabian Sea, receive some of the most frequent and heaviest rainfall during summer as well as being subject to extremes such as the 2018 Kerala floods.& Meanwhile, the Himalayas play a vital role in separating dry midlatitude flows from tropical airmasses and are subject to extremes during the summer monsoon, as well as in winter due to the passage of western disturbances.& This presentation summarizes the key results of IMPROVE.& Firstly, we examine the impact of orography on the observed convective diurnal cycle and assess its simulation in models at a range of resolutions including convection-permitting scales.& MetUM and WRF model experiments are used to identify key mechanisms and test their capability at simulating scale interactions between forcing at the large scale from the BSISO and newly identified regimes of on- and offshore convection near the Western Ghats.& An additional aspect to this work is the construction of a two-layer analytical model to test the behaviour of sheared flow perpendicular to a ridge analogous to the Western Ghats.& Secondly, the role of orography in extreme events is considered.& For the Western Ghats, this focuses on the interaction between monsoon low-pressure systems and the southwesterly flow in enhancing local rainfall.& For the Himalayas, we focus on characterising interactions between tropical lows and western disturbances in enhancing the orographic precipitation.& The work in IMPROVE works towards a deeper understanding of orographic rainfall and its extremes over India and uncovering why such mechanisms may be poorly represented in models.& &
Publisher: Copernicus GmbH
Date: 28-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-12069
Abstract: & & Regional orography around India exerts a profound control on weather and climate, both in summer and winter as part of the diurnal cycle of convection, as well as in extreme events. & This poster summarizes the key results of the Indo-UK IMPROVE project (Indian Monsoon Precipitation over Orography: Verification and Enhancement of understanding).& IMPROVE considers two focal regions.& The Western Ghats intercept the monsoon flow across the Arabian Sea and receive some of the most frequent and heaviest summer rainfall, including being subject to extremes such as the 2018 Kerala floods.& Meanwhile, the Himalayas play a vital role in separating dry midlatitude flows from tropical airmasses in summer, while suffering extremes in winter due to western disturbances - cyclonic storms propagating on the subtropical westerly jet.& & & & & We examine the impact of orography on the observed convective diurnal cycle and assess its simulation in models at a range of resolutions including convection-permitting scales.& MetUM and WRF model experiments, in addition to DWR retrievals, are used to identify key mechanisms between forcing at the large scale from the BSISO and newly identified regimes of on- and offshore convection near the Western Ghats.& An additional aspect to this work is consideration of a novel Froude number approach for understanding the convective regimes.& Secondly, the role of orography in extreme events is considered, including its interactions between passing tropical depressions or western disturbances.& Finally, land-atmosphere interactions occurring during the diurnal cycle of precipitation in the Western Ghats and Himalayas regions are discussed.& IMPROVE works towards a deeper understanding of orographic rainfall and its extremes over India and uncovering why such mechanisms may be poorly represented in models.& &
Publisher: American Geophysical Union (AGU)
Date: 04-2023
DOI: 10.1029/2022MS003537
Abstract: We test the application of a rare event simulation (RES) algorithm to accelerate the s ling of extreme winter rainfall over Europe in a climate model. The genealogical particle analysis algorithm, an ensemble method that interrupts the simulation at intermediate times to clone realizations in which an extreme event is developing, is applied to the intermediate complexity general circulation model PlaSim. We show that the algorithm strongly reduces the numerical effort required to estimate probabilities of extremes, demonstrating the potential of RES of seasonal precipitation extremes.
Publisher: Wiley
Date: 08-06-2020
Location: United States of America
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Reinhard Schiemann.